1. Field of Invention
The present invention relates generally to materials used for construction products. More particularly, the present invention relates to synthetic products made with improved features for a wider range of construction applications.
2. Description of the Prior Art
Construction members are available in a variety of natural, synthetic and composite materials, each of which has advantages and disadvantages depending on the specific application. Pressure-treated wood, for example, is economical and plentiful and lasts many years when properly and routinely treated with water repellent. Unfortunately, wood lumber can crack and splinter as it dries, change color over time and the sawdust can adversely impact human health and the environment depending on the compounds used to treat the wood.
An alternative to wood lumber is a composite material made from wood and plastic. For example, U.S. Pat. No. 6,153,293, Extruded Wood Polymer Composite and Method of Manufacture, describes extruded composite artificial lumber products manufactured from wood fiber and polyethylene. That patent also cites and discusses other patents disclosing structural members made from composite wood and plastic. Composites made of wood and plastic exhibit good strength, having a modulus of elasticity close to 175,000 pounds per square inch (ASTM D4761), they are economical to make, can be stained like wood and provide a relatively high coefficient of friction for mitigating slips. Moreover, structures made from composite materials resist rot, insects and ultraviolet radiation, are splinter free and easy to work with. In addition, the coefficient of thermal expansion for composites can be nearly half that compared to structural members made from just plastic. Some of the disadvantages of composite wood/plastic materials include a plastic appearance, colors can fade and the sawdust is not biodegradable. Further, composite materials may not be as strong as wood lumber and require closer joist spacing for decks. Carefree Composite® products, available from U.S. Plastic Lumber, are examples of composite structural members made from wood and recycled plastic.
Another alternative to wood lumber is a composite material made from plastics and certain chemical additives. For example, U.S. Pat. No. 5,030,662, Construction Material Obtained from Recycled Polyolefins Containing Other Polymers, describes composite artificial lumber products made from recycled polyolefins with additives such as reinforcing agents, impact modifiers and other materials. Depending on the plastic and additives used, they can exhibit good strength, having a modulus of elasticity (MOE) approaching 400,000 pounds per square inch (ASTM D198). They are also significantly stronger than just pure plastic materials without additives. However, composites made from recycled plastics and various chemical additives are generally more expensive than composites made from wood and plastic materials. Further, the strength of many composite plastic materials is lower than wood timber in good condition and, therefore, requires closer supports or joist spacing for decks. TRIMAX® products, also available from U.S. Plastic Lumber, are examples of structural members made from composite recycled plastics with chemical additives.
Notwithstanding the above, there are many applications where pure plastic structural members are desired because of certain advantages over other materials used for structural members. One of the primary materials used to manufacture artificial dimensional lumber products, for example, is high-density polyethylene (HDPE). Virgin HDPE is manufactured from petroleum fractions and is a relatively low-cost material, making it an attractive substitute for wood, composite wood/plastic and composite plastics/additives materials. Recycled HDPE is even more cost effective to use due in part to the availability of large quantities of the recycled materials.
Because HDPE products are made with a single, purified polymer, they can be manufactured to exacting, reproducible specifications. HDPE is also easily molded or extruded into a variety of shapes, including planks, posts, rails, balusters, and tongue and groove dimensional lumber. Construction products made with HDPE have exceptional resistance to corrosive substances, oil and fuels, insects, fungi, salt spray, and other environmental stresses. Moreover, HDPE products do not absorb moisture; therefore they will not rot, splinter or crack under most conditions.
Furthermore, HDPE can be colored with pigments, which are mixed with the HDPE before it is molded or extruded into its final dimensional lumber form, making it attractive for certain residential, commercial and industrial applications such as flooring planks on outdoor decks. Moreover, HDPE is reasonably strong at the temperatures to which it is ordinarily exposed.
Notwithstanding those advantages of HDPE as a material for construction members, HDPE has a lower modulus of elasticity and therefore less stiffness than wood lumber. For example, HDPE has a coefficient of thermal expansion of about 5.5×10−5 to 6×10−5 inches/inch-° F. That can translate into a change of length of a 10-foot deck plank of about one inch over a temperature change from 0° F. to 140° F. (the upper end of that range being observable in a product directly exposed to summer sun). Such significant length changes in a structure can result in excess stress on mounting members and fasteners, and warping and misalignment of structures employing such members.
Also, HDPE can discharge static electrical charges. Additives like Atmer® 129 by SpecialChem Company can be added to HDPE to mitigate the static electricity problem. In addition, HDPE loses over 30-percent of its flexural strength when heated from 70° F. to 120° F., thereby lowering its allowable live load. That requires the use of significantly larger structures or use of smaller spans (i.e., joists spaced closer together) compared to wood products. Moreover, HDPE is relatively slippery when wet, based on its coefficient of friction. Adding longitudinal (and traverse) ridges during the extrusion process, which has been a traditional method of increasing the coefficient of friction on the product surface, can generate an exaggerated risk of slides and falls under certain circumstances.
Accordingly, HDPE it is not always recommended for use as a true structural member in many applications. For example, HDPE by itself is not recommended for use in load-bearing walls, deck framing and floor joists. To overcome some of those problems, additives have been mixed with HDPE to improve it properties, included the addition of acrylics that increase notch sensitivity to fracture.